DOS ain't dead

From 0 to 1 MB in DOS / Beyond the 1 MB barrier in DOS (Developers)

posted by kerravon, Ligao, Free World North, 14.02.2024, 12:53

> > So if you have a CGA at 0xb8000 - before you do the
> > BIOS call that writes to the screen, bank the CGA
> > back in.
> >
> > That also required cooperation from the applications
> > though - to not manipulate the hardware directly.
>
> And no higher prio interrupts (timeslicers) that could potentially
> interfere. So that means applications can't hook (certain?) hw interrupts
> either.
>
> But since the whole dos compatibility essentially hung on the direct
> hardware access, without it would have been a different OS.

More - different applications - ones that conform to the rules.

The rules are actually here:

https://en.wikipedia.org/wiki/Protected_mode

If an application utilized or relied on any of the techniques below, it would not run:

Segment arithmetic
Privileged instructions
Direct hardware access
Writing to a code segment
Executing data
Overlapping segments
Use of BIOS functions, due to the BIOS interrupts being reserved by Intel

In reality, almost all DOS application programs violated these rules


But that to me is the fun thing - to start obeying the rules.

And for what I want to do (run microemacs) - I don't see a barrier to obeying the rules.

Although I wish to change the OS (to a combination of PDOS-generic and OS/2 1.x) for other reasons (mainly that "segment arithmetic" thing - I need something to provide AHINCR and AHSHIFT).

BFN. Paul.

Complete thread:

• From 0 to 1 MB in DOS / Beyond the 1 MB barrier in DOS - rr, 13.02.2024, 21:51
  • From 0 to 1 MB in DOS / Beyond the 1 MB barrier in DOS - kerravon, 14.02.2024, 11:51
    • From 0 to 1 MB in DOS / Beyond the 1 MB barrier in DOS - marcov, 14.02.2024, 11:59
      • From 0 to 1 MB in DOS / Beyond the 1 MB barrier in DOS - kerravon, 14.02.2024, 12:53